The present invention relates to thermal energy storage systems using liquids and, more particularly, to a method and apparatus for establishing a relatively narrow thermocline in a cooling liquid thermal energy storage tank.
Commercial cooling systems such as are used in larger buildings typically employ a liquid, usually water, although the water may be adulterated with various chemical compounds, as a heat transfer medium. A typical cooling system 10 of this type is shown in FIG. 1. A building load, such as a heat exchanger 12, transfers heat from a building to water circulating through the heat exchanger. The water enters the heat exchanger 12 from conduit 14 at a temperature of about 42.degree. F. Water exits the heat exchanger 12 through conduit 16 at a temperature of about 58.degree. F. A variable speed pump 18 in conduit 14, pumps water through heat exchanger 12 at a flow rate which is adjustable to maintain the water exiting the heat exchanger at the desired temperature of 58.degree. F. At the opposite end of the system, the water in conduit 16 is pumped by pumps 20 and 22 through corresponding ones of a pair of chillers 24,26 of a type well known in the art. The pumps 20,22 are constant flow pumps for maximum efficiency. If the rate of flow required by the building load for maintaining the exiting water temperature at 58.degree. F. is less than the available volume from the pumps 20 and 22, the excess flow is directed into a thermal energy storage or water tank 28 which acts as a primary/secondary system decoupler, the pump 18 and building load 12 being the secondary system while the pumps 20,22 and chillers 24,26 constitute the primary system.
During the normal cooling cycle, i.e., when cool water is being produced by chillers 24,26, the excess flow is pumped into the lower portion of tank 28. Since the system has a constant flow rate, an equal volume of water is withdrawn from tank 28 and returned to conduit 16. However, the water is withdrawn from the top of tank 28. If the water within tank 28 is properly handled by the tank system, the water will stratify so that cooler water remains in the lower portion of the tank and warmer water collects in the upper portion of the tank. Preferably, the water in the upper portion of the tank is at the desired 58.degree. F. temperature of water exiting the building load. This temperature is generally established by use of the tank 28 as the primary source of cooling water for building load 12 during a period of time when the chillers 24,26 and pumps 20,22 are taken off-line.
It will be appreciated that providing cooled water for building load 12 represents a significant energy load for an electric utility power company. As the number of such buildings has increased and placed a strain on the capacity of utility power companies, the companies have offered incentives and penalties to induce users to reduce electric power requirements, at least during peak demand hours. For cooling systems, peak demand generally runs from about noon to six p.m. During these hours, commercial users rely on cooling water stored in tank 28. The cool water (42.degree. F.) is withdrawn from the bottom of tank 28 into conduit 14, circulated through building load 12 and returned to the top of tank 28. This cycle stores 58.degree. F. water in the top of tank 28, which water is pulled off the top during the previously described normal cooling cycle. Although the chillers 24,26 are most efficient when converting 58.degree. F. water to 42.degree. F. water, they can maintain the 42.degree. F. output flow for lower inlet water temperatures using a conventional throttling control.
Each tank 28 for a building cooling system is designed to have a volume which matches the cooling requirements of the associated building. For example, assuming that the system must operate for six hours from tank 28, the volume of the tank is selected to provide sufficient 42.degree. F. water for the building load 12 for a six hour period. The tank 28 may be forty feet tall with a diameter of thirty-six feet. When water is stored in this tank, it is important to maintain separation between the 42.degree. F. water and the 58.degree. F. water. Stratification of water to achieve this result is well known. The warmer water is separated from the cooler water by a thermocline, i.e., an intermediate layer of water separating the warmer and cooler water. The thinner the thermocline, the more water that can be stored at the desired temperature. Any turbulence within the tank promotes mixing and increases the thickness of the thermocline. In some instances, the thermocline may be three or more feet thick and substantially reduce the volume of usable cooling water in the tank 28. Accordingly, it is desirable to provide a system which reduces the thermocline thickness.